In wireless communications, the IEEE 802.11 family of standards relating to “Wi-Fi” is now a popular system for allowing devices to communicate wirelessly using radio wave transmission. Groups of devices all communicating via a common wireless access gateway are known as wireless local area networks (WLANs).
Wi-Fi communication is licensed to operate in two sections of radio spectrum, 2.4 Ghz and 5 Ghz. At present, most devices are only able to operate in the popular 2.4 Ghz spectrum band, while more recent devices can also operate in the 5 Ghz band which is generally less congested.
In 2.4 Ghz Wi-Fi, the spectrum between 2402 Mhz and 2472 Mhz has been divided into a number of channels, each channel has a central frequency spaced 5 Mhz apart from its neighboring channel. However there is spectra overlap such that transmissions on channel 1 will interfere with transmissions on channel 2,3,4,5, while transmissions on channel 2 will interfere with transmissions on channels 1 and 3,4,5,6, etc.
Since there is no central controller for handling collisions in Wi-Fi, WLAN devices use a protocol known as Carrier Sense, Multiple Access with Collision Avoidance (CSMA) so that only a single device is transmitting at any one time.
CSMA is a listen-before-talk approach that attempts to determine whether any other transmitter is operating in the Wi-Fi band. In addition Wi-Fi devices uses a variety of proprietary modulation rate adaption algorithms that varying the modulation and coding in accordance with the Signal to noise ratio of the channel. Both CMSA and rate adaption are dependent on the received noise level which includes interference from other non-Wi-Fi transmitters.
In particular, the recent emergence of LTE femtocells operating in the 2.3 GHz and 2.6 Ghz spectrum bands has resulted in the potential for more interference in the form of out of band interference that can cause subtle effects on Wi-Fi performance and range that are not easily detected by the Wi-Fi devices and which can affect Wi-Fi devices in different ways. For example, out-of-band interference can interfere with the automatic gain control of Wi-Fi receivers resulting in reduced sensitivity.
For example, LTE Band 7 and Band 40 lie on either side of the 2.4 Ghz Wi-Fi spectrum with relatively small frequency guard bands of <30 Mhz. For some Wi-Fi receivers, in particular older hardware released when the surrounding spectrum was not generally used, having poor out-of-band rejection, LTE transmissions in these bands can result in adjacent channel interference affecting Wi-Fi throughput. The main effect is to cause a reduction in the Wi-Fi receiver sensitivity resulting in reduced Wi-Fi range and reduced throughputs at range. In a worse case, i.e. where the LTE transmit power is high, e.g. >15 dBm and the LTE transmitter is close to the Wi-Fi station (e.g. <1 m) then the LTE transmissions can raise the noise floor above the carrier sense threshold such that the Wi-Fi transmitter cannot perceive the channel as idle and so can never transmit which may result in disruption to, and possible disconnection of the Wi-Fi link.